1. Field of the Invention
This invention relates to a method of forming a metal material in a die assembly, and more particularly to a die-forging of a semi-solidified metal composition as a starting material at a solid-liquid coexistent temperature region.
2. Description of the Related Art
In general, there are various methods of forming a metal material, among which a forming method such as press forming or the like is widely used for the formation of structural parts. In press forming, the metal material has hitherto been shaped at a temperature below solids, but such a method has several problems cracking is apt to be caused when forming complicated parts or hardly workable parts, a large working load is required, plural forming steps are required, and the like. In order to provide the parts of a given shape, it may be obliged to adopt another method such as forging or the like even if the properties of the resulting part are poor.
As a countermeasure for solving the above problems, there has been developed a method wherein the material is formed at such a state that the material temperature is approximately equal to the die temperature under particular working conditions, or a so-called isothermal forging method. This isothermal forging method can reduce the mechanical working cost for finishing into a final shape when forming a hardly workable material and also effectively contributes to decreasing the working load and the like.
However, the isothermal forging method requires precise control over working rate, so that it has a problem that the equipment for conducting this method becomes too large.
In order to solve the aforementioned problems and widen the range of materials to be formed, a method of working metal in a temperature region between solids and liquids or a solid-liquid coexistent temperature region has recently been studied in various fields. As an example of this method, a method wherein the metal is agitated at the solid-liquid coexistent temperature region by a mechanical means or the like to form non-dendritic structure or a granular structure and solidified at once to form a working material and then the working material is again heated to the solid-liquid coexistent temperature region for forming is disclosed in U.S. Pat. No. 4,771,818.
In general, the method of working the metal at the solid-liquid coexistent temperature region is advantageous for forming hardly workable material, complicated parts or the like because the fluidity of the metal material is good and the force required for the working is small.
However, the above working method has a problem which has never been observed in the conventional techniques.
That is, since the metal is formed at the solid-liquid coexistent temperature region, when the metal material is filled in a die assembly at the forming step, solid phase and liquid phase flow ununiformly and hence the ununiform distribution of solid phase and liquid phase or macrosegregation is caused in a section of the resulting formed product at the completion of the forming. As such a segregation is caused, the structure of the product section becomes ununiform and hence the mechanical properties of the product are ununiform, which are harmful in practical use.